The thick film circuit is one well-known form of monolithic integrated microelectronic circuit. Circuits of this type are particularly useful where a large number of passive components are required, or where moderately high power dissipation is required. Thick film circuits are less costly to produce and can yield a broader range of resistance values than thin film circuits.
The manufacture of thick film circuits is a refinement of the well-known art of silk-screen printing. Thick film circuits consist of patterns of conductors, resistors and other passive circuit components printed on a particular substrate. In most known processes, a variety of pastes are applied onto a substrate or successive circuit layers through a screen or a template of a specific printed pattern. The successive layers are dried after printing and fired in a belt furnace to sinter the material.
In a typical thick film circuit, the substrate is often a ceramic material, such as alumina. However, for demanding applications such as in automotive electronics where protection against breakage due to vibrations is required, glass coated metallic such as stainless steel substrates are used. In these applications there is much room for improvement. Thick film pastes are typically compositions of glass particles, metal and/or metal oxide particles, together with organic solvents, resins and viscosity control agents known as thixotropes. The compositions of these thick film pastes depend upon the type of passive electrical component being printed.
A variety of metal-containing thick film compositions (i.e., pastes, inks, tapes, etc.) useful in forming resistors, dielectrics and conductors which are employed in hybrid microelectronic components have been developed in the field of hybrid microelectronics. Generally, such compositions, and particularly paste or ink compositions, include a conductor (e.g., silver, palladium, copper, aluminum, gold, platinum, and the like as well as alloys of each of these different metals), resistive or dielectric components, a binder or an inorganic fluxing material, (e.g., a glass or inorganic oxides), and a carrier or vehicle comprising generally a solvent with a resin and a thixotrope and/or a wetting agent.
The above-described paste or ink compositions are applied in the desired configuration or pattern onto a suitable substrate to form the desired circuit for use as a hybrid microelectronic component. There have been a number of substrate materials developed for use in these applications. For example, such traditional substrate materials may include alumina (Al2O3), glass coated metal, barium titanate (BaTiO3), beryllia (BeO), aluminum nitride (AlN), and silicon carbide (SiC).
The prior art sought to achieve desirable thick film properties in various ways. The introduction of cadmium and lead to the prior art glass compositions provided important characteristics such as moderate linear coefficient of expansion, improved solder leach resistance, and good chemical durability compared to glasses containing high concentrations of alkali oxides, and the ability to fire these thick film paste compositions at fairly low temperatures. The inorganic fluxing materials especially the glass compositions in the thick film pastes impart many characteristics such as adhesion to substrates, solder leach resistance for the thick film compositions. A glass composition comprising PbO is illustrated, for example, in the patent to Hormadaly, U.S. Pat. No. 5,114,885. It is known that the use of PbO as an ingredient in a glass component of a conductive film tends to lower the firing temperature of these thick film compositions and produce a coating that has a superior surface finish. For this and other reasons, PbO and CdO were significant components in many prior art thick film glass compositions. However, in light of environmental concerns, the use of PbO as well as of CdO, in thick film or glass enamel compositions is now largely avoided whenever possible. Hence, a need exists in the electronics industry for thick film compositions, which afford desirable properties using lead free and cadmium free glasses in thick film pastes.